The invention relates generally to monitoring the delivery of fluid through a fluid delivery system, and more particularly, to detecting the position of a cannula used in delivering fluid to a patient's blood vessel.
Fluid delivery systems for intravenously infusing fluid to a patient typically include a supply of the IV fluid, an infusion needle or cannula, an IV administration set connecting the fluid supply to the cannula, and a positive displacement infusion pump. The cannula is mounted at the distal end of the flexible tubing of the IV administration set for insertion into a patient's blood vessel to deliver the fluid infusate to the patient. One commonly used infusion pump is a peristaltic type having several rollers, cams or cam-actuated fingers which sequentially occlude portions of the flexible tubing along a pumping zone to create a moving zone of occlusion. The peristaltic action forces the IV fluid through the tubing of the administration set to the cannula and into the patient.
During infusion, conditions such as phlebitis and/or infiltration may develop if the cannula is allowed to press against the vessel wall continuously. Phlebitis is the inflammation of the vessel wall due to the action of the undiluted infusate directly contacting the vessel wall. Infiltration is a condition in which infused fluid finds its way into extravascular tissues rather than being released solely into the blood stream. When this occurs, fluid may be infused into the interstitial spaces between layers of tissues. Not only is the patient deprived of proper intravenous drug administration, but the patient is further subjected to the possible toxic or caustic effects associated with the infused fluids directly contacting body tissue.
A commonly used manual clinical technique for evaluating catheter patency and for detecting infiltration involves an attempt to aspirate a small volume of blood from the cannula. This is implemented either by attachment of a syringe to an injection site in the IV line or by use of a device commonly referred to as a "flashball" which is a component of the IV administration set itself. After introduction of the cannula into the blood vessel, a small volume of IV fluid is first infused by squeezing the ball, and then releasing it. If blood is aspirated into the cannula and the IV set, then patency of the cannula is assumed. Aspirated blood must be reinfused with a rapid flush in each method.
However, the manual methods have several disadvantages including: risk of contamination; non-continuous monitoring; interruption of therapy, which is a particularly acute disadvantage when high potency, rapid acting drugs are infused; the potential for clot dislodgement; and potential for vessel damage and inconsistent technique and judgement criteria.
Methods have been developed for detecting the existence of an extravascular infiltration; however, such methods including the above-described manual method are generally directed at detecting an infiltration after it has occurred. These methods are typically not sensitive to cannula positional irregularities where the cannula remains in the patient's vessel but has moved into a position from which it may eventually progress to an infiltration. For example, a cannula that is inside the vessel but which has gradually moved into a position such that its output port is in contact with the vessel wall or at least directs the IV solution directly against the vessel wall may result in vessel wall damage leading eventually to phlebitis and/or infiltration as the vessel wall intima is irritated. Should this position of the cannula be detected early enough, infiltration may be avoided. Hence, those skilled in the art have recognized that it would be beneficial to detect a cannula which has moved against the vessel wall before substantial damage to that wall has occurred.
Other systems have been devised for the detection of abnormalities in an intravascular infusion system by use of simple monitoring of the infusion pressure. A high or increasing pressure may be interpreted as either an infiltration or an occlusion. A low pressure may be interpreted as an unobstructed line or a line that has completely withdrawn from the vessel. However, the pressure developed at a given flow rate in an intravascular system depends on many factors, such as motion and position of the patient, respiration, arterial or venous blood pressure of the patient, and the size and position of the cannula used. Such factors create a considerable uncertainty in the measurement of pressure, and may cause difficulty in interpretation of pressure readings.
Observation of the pressure response of a fluid delivery system to a flow pattern to determine when the pressure returns to a normal steady state equilibrium can give some information as to the effectiveness of fluid delivery. Application of fluid flow excitation volumes and the comparison of pressure responses of the system against a reference value are also known. However, the effectiveness of such techniques is lessened by patient movement, respiration, arterial or venous blood pressure, and other factors.
It has also been proposed in the art that infiltration can be detected by identifying a substantial lack of symmetry between the pressure response time to a positive fluid pulse and the pressure response time to a negative fluid pulse. In such a system, the pressure response to each pulse is monitored to determine the point in time that the pressure response reaches a predetermined reference pressure. The accuracy of such as system can be impacted by the existence of artifacts. For example, movement of the patient during the time that the pressure response is monitored can result in the pressure reaching the predetermined reference pressure prematurely, causing inaccuracy.
Hence, those skilled in the art have recognized a need for a cannula position detection system which can detect an undesirable cannula position prior to an infiltration and which can continuously monitor the position without interrupting therapy. Additionally, it has also been recognized that there is a need for a system that is relatively insensitive to patient movement and other factors which may affect the monitoring of the infusion. The present invention fulfills these needs and others.